The present invention relates to a device for measuring gas permeability through a membrane; more particularly, the invention is an improved gas permeability measuring device which permits such measurements to be accomplished at substantially constant temperature and relative humidity.
Gas permeability measuring devices are generally known in the prior art, including a number of such devices which are manufactured by the assignee of the present invention. Such devices typically include one or more sensing heads which are adapted for holding a membrane material across a chamber, wherein a gas such as oxygen may be admitted into the chamber on one side of the membrane, and a detector such as an oxygen detector may be coupled via passages to the other side of the chamber, to measure the amount of oxygen which passes through the membrane. Since all membranes are permeable to some extent, it is usually possible to detect a measurable amount of oxygen passing through the membrane over a finite period of time. In the prior art, gas permeability measuring devices utilized one or more of such measuring heads coupled via hoses and tubing to sensors and the like, to perform fairly accurate measurements of membrane permeability.
Measurement of gas permeability through membranes requires extremely sensitive gas detectors or sensors, for the quantities of measured gas are frequently quite low. It is therefore extremely important that the entire system involved in such measurements be maintained under tightly sealed conditions, particularly with respect to all of the gas flow passages leading to the gas detector. Prior art permeability measuring instruments typically utilize hoses or tubing to interconnect the necessary instrumentation, wherein each of the connecting junctions is susceptible to leakage. Since the performance of these instruments can be critically degraded by gas leakage, it is important to the design of such instruments to provide a minimum number of connections in the gas flow path.
It is also known in the prior art to construct gas permeability sensors operating under various conditions of relative humidity of the gas. Relative humidity becomes an important factor in measuring gas permeability through membranes, because the permeability of certain membranes is affected by the relative humidity of the membrane and the surrounding gas. Measuring gas permeability under conditions of high relative humidity is exceedingly difficult to accomplish, because relative humidity and temperature are closely interrelated, and it therefore becomes necessary to maintain precise control over temperature if permeability is to be measured under relatively high humidity conditions. Under these conditions it is necessary to control temperature of all of the gas flow paths in the system, for a 1.degree. C. change in temperature can easily result in a 5% change in relative humidity. Furthermore, under high relative humidity conditions a slight decrease in temperature can cause immediate condensation of the gas, resulting in liquid accumulation in the gas flow passages. Therefore it becomes extremely important to control the temperature of the entire measurement system whenever permeability measurements are desired with respect to humid gases.
Among the systems devised in the prior art for measuring permeability are a line of products manufactured by the assignee of the present invention under the general model designation "OX-TRAN." These systems have proved very effective for measuring gas permeability under widely varying conditions, although permeability measurement under high humidity conditions have necessitated relatively expensive and complex improvements to the basic system models. Examples of patented prior art technology can be found in U.S. Pat. No. 3,590,634 "Instrument for Determining Permeation Rates Through a Membrane," which describes a simple permeation measuring device utilizing dry gases. U.S. Pat. No. 4,464,927 "Apparatus for Measuring Gas Transmission Through Films," Aug. 14, 1984, describes another simple measuring device utilizing multiple permeation cells. U.S. Pat. No. 4,852,389 "System for Controlled Humidity Tests," Aug. 1, 1989, discloses a gas permeability measuring device capable of operating under different conditions of relative humidity in the gas. This last patent illustrates the complexity of equipment which has been necessary in order to accurately accomplish permeability measurements under conditions of controlled humidity and temperature.